Fiber-reinforced composite materials comprising carbon fibers and matrix resin are excellent in mechanical properties with a high specific strength and a high specific elastic modulus and also have high functional properties such as weatherability and chemical resistance, so that they have attracted attention to be demanded as materials for aircrafts and general industrial use.
Patent document 1 discloses a prepreg as a fiber-reinforced composite material which has been made from resin and unidirectionally-oriented fiber base material made from non-crimp continuous carbon fibers. Such a composite material is excellent in mechanical properties with a high fiber volume content since the fibers are packed densely as oriented unidirectionally. Further, since it can be designed to have required mechanical properties precisely with little unevenness in mechanical properties, it has been variously applied to aircrafts, etc.
However, it is known that fiber-reinforced composite materials can exhibit only a unidirectional strength along the fiber orientation. Therefore, a piece of such a composite material provides an in-plane anisotropy which exhibits a high strength only in a direction along the unidirectional fiber orientation. In order to ensure an isotropy of mechanical properties, a plurality of composite materials can be laminated so that they are oriented variously, such as along a fiber orientation, perpendicularly to the fiber orientation and obliquely to the fiber orientation. Such a laminated material requiring additional costs tends to form a thick product which exfoliates between layers easily relative to the one not laminated.
Patent document 2 discloses a chopped strand prepreg comprising resin and carbon discontinuous fibers having number average fiber length around 15 to 45 mm. Such a prepreg in which fibers are randomly oriented can exhibit a quasi-isotropy without laminating the composite materials.
However, it is difficult to obtain a composite material having a high fiber volume content since the randomly-oriented rigid fibers impregnated with resin cannot be packed densely.
On the other hand, a process for producing an entangled carbon fiber nonwoven fabric is known as being applicable to an electrode base material for sodium-sulfur batteries other than fiber-reinforced composite materials impregnated with resin. Although it is difficult to entangle rigid carbon fibers, an entangled carbon fiber nonwoven fabric can be obtained by burning entangled polyacrylonitrile flame-resistant yarns as a precursor of polyacrylonitrile-based carbon fibers, as disclosed in Patent document 3.
On the other hand, in order to entangle fibers or give cushion characteristics and bulkiness to a nonwoven fabric applied to cushion material, mat or the like, fibers have been crimped conventionally.
Such a battery electrode base material focused on conductivity of carbon fibers has not been required to be strong so much. Patent document 4 discloses that carbon fibers used for a fiber-reinforced composite material should not be crimped so as not to be mechanically damaged to deteriorate the carbon fibers. Therefore, it is not usual that carbon fibers used for fiber-reinforced composite materials are positively entangled or crimped, in case that the strength decreases.
Patent document 5 discloses a technology that acrylic fibers as a precursor of flame-resistant fiber are crimped and then treated to be flame-resistant without any tension, and then are burnt to make carbon fibers.
Although it is important in practical study what kind of structure is formed with fibers, Patent document 5 only discloses that it is suitable for various electrode base materials while it doesn't disclose concretely about any composite material made from nonwoven fabric which has been entangled with the carbon fibers and impregnated with resin. It discloses neither quasi-isotropy nor high fiber volume content.